HUNTINGTON, Utah (CNN) -- Some of the miners at Utah's Crandall Canyon mine -- including one of the men trapped by Monday's cave-in -- apparently were concerned about working in the area of the collapse, a source told CNN.

The source, who requested anonymity, said the six trapped miners were working in an area called 7 Belt -- the deepest part of the mine.

A large hole being drilled in an effort to reach the men had bored through 1,644 feet of earth as of Friday night, officials said. A total depth of 1,886 feet is anticipated, said Richard Stickler, assistant secretary of the Department of Labor for mine safety and health.

In recent weeks, the floors in that part of the mine had been "heaving," or buckling up, from intense pressure, said the source, who has intimate knowledge of the conditions in the mine.

Supervisors at the mine knew of the problem, he said.

Several miners -- reportedly including Manuel Sanchez, who is among the trapped men -- were becoming apprehensive, the source said.

A member of Sanchez's family told a Utah newspaper that he had expressed concern about safety in one part of the mine.

The mine's operator said he was not aware of the safety concerns.

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"I've never heard that," Bob Murray, president and CEO of Murray Energy, told CNN's Ted Rowlands when asked why someone would have been worried about that section of the mine. "I have no idea. It's probably a rumor, and I'm not going to respond to rumors."

Sanchez has been trapped since early Monday, along with five other miners. The mining company has not released the miners' names, but family and friends have confirmed to CNN the identities of Sanchez, Kerry Allred, Carlos Payan, Brandon Phillips, Alonso Hernandez and Don Erikson. Watch a report on miner safety concerns »

Asked why they did not complain about their safety concerns, several miners said complaining means the loss of a job.

Murray denied that. "If you're getting that from the community, then those miners must work for another mining company. I don't operate that way," he said.

Not so, said Paul Riddle, who used to work in one of Murray's mines. "Always profits before safety, that's my opinion, my feeling, my experience," he said.

Miners who work for Murray are sometimes forced to push the envelope when it comes to safety, he said, and are afraid to speak up for fear of being fired.

"I'm not the only one," he said. "There are many, many people that feel this way and are afraid to speak up."

An attempt to make contact with the trapped miners was unsuccessful Friday, when a microphone lowered into a narrow hole drilled into a mine cavity detected no sounds. Murray said that technical difficulties prevented the microphone from picking up sound, the drill steel was blocking the microphone, and a line broke.

Survey equipment showed that the drill drifted 87 feet during the drilling but ended up in an active area of the mine, where the miners could be, Stickler said. There has been no communication with the miners since the collapse.

Air tests overnight suggested there may be oxygen levels of about 20 percent in the area where the miners are thought to be, Stickler said. Later tests found oxygen levels at about 7 percent -- too little to survive -- but that may be because the drill hole drifted as it was being bored, Stickler said. See where the miners are thought to be trapped »

"The fact that we have not picked up any sound I believe should not be interpreted as bad news," Murray said.

"There could be a number of factors as to why sounds in there might not be picked up, and I wouldn't look at it as good or bad news," he said.

A second, larger hole being drilled into the mine has a better chance of accuracy, Stickler said. That drill -- more than 8 inches in diameter -- should reach the cavity late Friday or early Saturday, he said.

As of Friday evening, the larger drill was about 240 feet away from the miners' presumed location, Stickler said.

"We feel that we will have a better chance of maintaining that hole [the larger one] and put a TV camera down it," Murray said.

Murray said Thursday that if the miners survived the collapse, they could continue to live on fresh air, food and water supplied through the holes until crews can remove tons of coal and rock that clogged a collapsed tunnel.

Murray has insisted a magnitude-3.9 earthquake caused the mine collapse, and has said at least 10 "aftershocks" have been recorded, with seismic activity earlier wiping out more than 300 feet of progress and halting rescue efforts temporarily.

Seismologists and geophysicists have not been as sure, saying the seismic activity they measured appeared to stem from the mine's collapse.

On Thursday, University of California-Berkeley seismologist Douglas Dreger said the data show the shaking detected bore the signature of a collapse and "not a tectonic earthquake."

Experts have said the "aftershocks" could be the rock adjusting after the collapse.